KVM: PPC: Book3S HV: Add capability to report possible virtual SMT modes

Now that userspace can set the virtual SMT mode by enabling the
KVM_CAP_PPC_SMT capability, it is useful for userspace to be able
to query the set of possible virtual SMT modes.  This provides a
new capability, KVM_CAP_PPC_SMT_POSSIBLE, to provide this
information.  The return value is a bitmap of possible modes, with
bit N set if virtual SMT mode 2^N is available.  That is, 1 indicates
SMT1 is available, 2 indicates that SMT2 is available, 3 indicates
that both SMT1 and SMT2 are available, and so on.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

Documentation/virtual/kvm/api.txt

@@ -4010,6 +4010,8 @@ be 0.  A successful call to enable this capability will result in
 vsmt_mode being returned when the KVM_CAP_PPC_SMT capability is
 subsequently queried for the VM.  This capability is only supported by
 HV KVM, and can only be set before any VCPUs have been created.
+The KVM_CAP_PPC_SMT_POSSIBLE capability indicates which virtual SMT
+modes are available.
 
 7.12 KVM_CAP_PPC_FWNMI
 
@@ -4183,3 +4185,12 @@ Currently the following bits are defined for the device_irq_level bitmap:
 Future versions of kvm may implement additional events. These will get
 indicated by returning a higher number from KVM_CHECK_EXTENSION and will be
 listed above.
+
+8.10 KVM_CAP_PPC_SMT_POSSIBLE
+
+Architectures: ppc
+
+Querying this capability returns a bitmap indicating the possible
+virtual SMT modes that can be set using KVM_CAP_PPC_SMT.  If bit N
+(counting from the right) is set, then a virtual SMT mode of 2^N is
+available.

arch/powerpc/kvm/powerpc.c

@@ -566,6 +566,16 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 				r = threads_per_subcore;
 		}
 		break;
+	case KVM_CAP_PPC_SMT_POSSIBLE:
+		r = 1;
+		if (hv_enabled) {
+			if (!cpu_has_feature(CPU_FTR_ARCH_300))
+				r = ((threads_per_subcore << 1) - 1);
+			else
+				/* P9 can emulate dbells, so allow any mode */
+				r = 8 | 4 | 2 | 1;
+		}
+		break;
 	case KVM_CAP_PPC_RMA:
 		r = 0;
 		break;

include/uapi/linux/kvm.h

@@ -896,6 +896,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_X86_GUEST_MWAIT 143
 #define KVM_CAP_ARM_USER_IRQ 144
 #define KVM_CAP_PPC_FWNMI 145
+#define KVM_CAP_PPC_SMT_POSSIBLE 146
 
 #ifdef KVM_CAP_IRQ_ROUTING